Sunlight contains both UVA radiation, with a wavelength of 320-400 nm, and UVB radiation, with a wavelength of 280-320 nm. Both UVA and UVB radiation have detrimental cytotoxic effects on skin cells, but with different effect mechanisms. UVB radiation directly induces mutations and damages in the DNA of skin cells. As for UVA radiation, it induces the creation of reactive free radicals on the skin surface from coumarin-type compounds. Thereafter said free radicals can damage skin cells, thus predisposing the skin to various cancer diseases. The effect of UVB is inhibited by protecting the skin with sun protective compositions containing UV filtering agent that transfers UVB radiation into a form that is harmless to skin cells, such as heat or long-wave radiation. The effect of UVA radiation is inhibited by UV filters (UV protection agents) that are capable of trapping free radicals induced by sunlight.
Both inorganic and organic compounds are used in sun protective compositions as UV protection agents, to prevent the detrimental effect on skin cells of UV radiation contained in sunlight. They either absorb and reflect UV radiation within a relatively narrow wavelength range, or alternatively capture free radicals induced by UV radiation, such as oxygen, hydroxyl or peroxide radicals.
Inorganic UV filters (below also inorganic UV protection agents) are finely divided (10-100 nm) inorganic mineral pigments, such as TiO2, ZnO and FexOy, which reflect or absorb UV radiation.
Among the compounds used in organic sun protective compositions and serving mainly as UVB protection agents, absorbing UVB radiation, are for example octyl triazine, urocanic acid, octyl methoxy cinnamate, methylbenzylidene camphor, 3-benzylidene sulphonic acid and PABA i.e. octyl dimethyl, 4-aminobenzoic acid (para-aminobenzoic acid).
Typical UVA radiation inhibitors used in sun protective compositions, functioning by the free radical trapping mechanism, are organic aromatic compounds containing conjugated carbonyl groups. Among these are for example benzophenon-3, benzophenon-4 butyl methoxydibenzoylmethane and terephtalyliden sulphonic acid. Said aromatic compounds protect the DNA of skin cells by preventing conjugated DNA double bonds from absorbing UV radiation both within the UVA range (320-360 nm) and also within the UVB range (280-320 nm).
At present, there are also available organic UV protection agents that are capable of absorbing UV radiation both within the UVA range and the UVB range. This kind of agent is for example MBBT.
Antioxidants are often added in sun protective compositions in order to prevent the oxidation of the compounds contained in the composition and to capture the possibly created oxygenous reactive compounds and free radicals. Also in foodstuffs there are used antioxidants that are capable of neutralizing, i.e. trapping oxygenous free radicals created through the auto-oxidation of foodstuffs, such as peroxide radicals. Thus they slow down the auto-oxidation process of food, preventing changes in the color, smell and taste of food. Among these antioxidants, there are natural vitamins such as A, B, C, D and E vitamins, and synthetic vitamins such as the above mentioned analog of vitamin B, i.e. para-aminobenzoic acid, the synthetic phenolic analog of vitamin E, BHT, i.e. 2,6-(bis)-(1.1-dimethylethyl)-4-methylphenyl (butylated hydroxy toluene) and BHA (butylated hydroxy anisole, which is a mixture of two isomeric organic compounds, containing 2-tert-butyl-4-hydroxy anisole and 3-tert-butyl-4-hydroxy anisole. The antioxidants used in foodstuffs and sun protective compositions may in certain conditions be cytotoxic to the human organism. Among these, there are also BHT (foodstuff additive E321) and BHA, which means that their use is restricted, and they cannot be sufficiently added to the target of usage in order to ensure their antioxidative effect.
Certain organic UV protection agents or inorganic mineral compounds used for absorbing UV radiation may, however, spontaneously release free radicals owing to the effect of UV radiation, or they may induce the creation of free radicals in the target of usage. Among free radicals, let us mention for example hydroxyl radicals, peroxide radicals, unimolecular oxygen, and temporary lipid radicals. When penetrating to the human system, free radicals may cause cytotoxic exposure.
Consequently, compounds used in sun protection agents may, owing to the effect of UV radiation, spontaneously release free radicals. As was pointed out above, in sun protective compositions there are generally used inorganic UV protection agents TiO2 and ZnO, CeO2, which absorb UV radiation within a wide wavelength range. However, inorganic UV filters have a strong tendency to be agglomerated owing to the effect of electrostatic forces, which means that their efficiency is drastically reduced. Therefore said finely divided inorganic pigments must be coated with silica or aluminum compounds and dispersed in a solvent, such as dispersion oil, for example in isononyl-isononanoate or dimethicone, in the presence of suitable surface active agents. Coated inorganic UV-filter mediums may themselves turn cytotoxic; in aqueous conditions, they may induce the creation of hydroxyl radicals on the skin, in case their protective coating layer is worn off. They may also themselves turn into reactive radicals, in case there are holes in their coating, or if the coating is worn off.
In the course of time, several organic UV protection agents, such as PABA, OMC and Octocrylene, gradually penetrate into the skin. In case the sun protective composition is not reapplied on the skin, or if the layer applied on the skin is otherwise worn off, there are created monoatomic reactive oxygen radicals owing to the effect of UV light. It has also been found that benzophenon-3, a UV filter generally used in sun protection agents, also penetrates into the system through the skin, which means that it may itself induce the creation of free radicals deeper in the system.
It is often necessary to add microbicidic agents into cosmetic compositions and technical solvents and cleaning agents as well as industrially used solvents. Microbicidic agents may be cytotoxic or otherwise detrimental to the system when getting into contact with the skin.